1. Field of the Invention:
The present invention relates generally to video imaging systems, and in particular to a sweep generator for producing a circular raster of concentric curved traces on the target of a video camera.
2. Description of the Prior Art:
A widely used method for providing continuous surveillance over a predetermined area is by video transmission. The most elemental video surveillance system consists of a single camera permanently connected to a video monitor through a closed circuit transmission line. The video camera converts the virtual image of the area being scanned into electrical impulses which make up the video signal. In the surveillance of panoramic scenes it has been customary to use a single camera mounted on a support and capable of rotation about a vertical axis whereby a succession of images representative of the panoramic view are produced. In such systems the video camera is focused at successive points and therefore is incapable of viewing all points within the panoramic scene simultaneously.
There are a number of situations wherein it is desirable to provide simultaneous surveillance of a number of locations within one panoramic view. This objective has been accomplished by the use of multiple cameras and multiple receivers for covering various zones of interest. Complete acceptance and widespread use of the foregoing approach has been limited by the complexity and expense of equipment required to provide continuous surveillance of multiple locations. To minimize the equipment required, a panoramic optical lens has been developed which produces a virtual image of a panoramic view in the form of a continuous annular projection.
Although it is possible to scan the annular virtual image projected upon the target surface of a video camera according to a conventional horizontal scanning pattern, the image reproduced in the video receiver is an exact duplicate of the annular image and necessarily includes a central region of the raster surface of the video receiving tube which is unused. Additionally, in the annular image, a portion of the picture is inverted with respect to the panoramic view as normally observed. Therefore, in order to make efficient use of the raster surface of a single video picture tube, it is desirable to "spread" or transform the annular virtual image of the panoramic view into a rectangular image in which the complete panoramic view is portrayed without image inversion.
Various attempts have been made to sweep the scanning beam of the video camera along a spiral path and to supply video signals produced by the spiral scanning of the panoramic image to a video receiver for presentation in the horizontal lines of a rectangular raster. This approach has met with only limited success since the video signals corresponding with the portion of the virtual image scanned in the camera during the receiver retrace period are blanked out. Since the horizontal retrace period typically represents sixteen percent of the horizontal trace cycle, an angular sector measuring as much as 58.degree. of the panoramic view will be lost.